Confirming UK involvement in the attack in her speech, May said: "We would have preferred an alternative path. But on this occasion there is none."

She said the attacks were not about "regime change" or "intervening in a civil war", but were to "deter the use of chemical weapons" by the Syrian government.

After her statement, May was expected to ask the speaker of parliament to grant an emergency debate, followed possibly by a non-binding vote.

Al Jazeera's Jonah Hill, reporting from London, said: "I don't think Monday's session will satisfy the opposition parties, who are determined that they should have had, and could still retrospectively have, a definitive vote on the issue of the strikes themselves."